This invention relates to an orientation film for a liquid crystal display element employing a liquid crystal having a helical structure of which the twisted angle between substrates is in the range of 200.degree. to 250.degree.. More particularly, the present invention is concerned with a liquid crystal display element of the type indicated above which is free from scattering domain.
In conventional liquid crystal elements of a twisted nematic (TN) mode, liquid crystal molecule orientation has been regulated by means of an orientation film disposed on a pair of substrates with electrodes so as to be twisted by 90.degree.. Silicon monoxide (SiO) films obtained by the oblique evaporation method and polyimide-based materials have mainly been used as the orientation film. However, with the TN mode liquid crystal elements, the realization of a large-capacity display cannot be expected, because difficulties are encountered in attaining a great improvement in time sharing characteristics.
On the other hand, in recent years, a liquid crystal element which took advantage of a supertwisted birefringence effect (SBE) attained by orienting liquid crystal molecules into a 270.degree. twisting from one substrate to the other was reported at 1985 SID International Symposium held at Orlando Fla. U.S.A. on Apr. 30-May 2 (SID 85 DIGEST, pp. 120-123) and is expected as a liquid crystal element which may find a new field of applications for liquid crystals.
However, a liquid crystal element such as an SBE liquid crystal element in which the twisted angle of the liquid crystal modecules is e.g. 270.degree., much larger than 90.degree. which is adopted in the conventional TN mode liquid crystal elements, cannot successfully be combined with the conventional orientation films causing a small tilt angle which was subjected to orientation treatment by rubbing, because such a combination brings about a scattering domain (a domain in which light is scattered), which leads to a poor display quality. Until now no way has been found to prevent occurrence of the scattering domain except for making the tilt angle of the liquid crystal molecules by the orientation film larger. Although oblique evaporation is a means for making the tilt angle larger, it is unsuited for mass production of such an orientation film. Therefore, the realization of a film made of an organic polymer capable of causing a tilt angle larger than that by the conventional orientation film has been an important task to be attained.
In the conventional TN mode liquid crystal element which uses a nematic liquid crystal, the smaller the tilt angle by the orientation film, the better the time sharing drive characteristics. Therefore, the TN mode liquid crystal elements have been prepared using film materials which provide an orientation film causing a tilt angle of about 3.degree. or less. However, as opposed to the TN mode liquid crystal element, the SBE mode liquid crystal element had a problem of occurrence of a scattering domain due to a small tilt angle molecules caused by the orientation film.